Duct burner for operation with liquid or gaseous fuels

ABSTRACT

An air augmented liquid or gaseous fuel burner having a hollow post mounting a plurality of fuel discharge nozzles spaced over the length of the post and a combustion chamber aligned with each discharge nozzle. An air plenum is disposed about the nozzle, the post and the combustion chamber. An upstream end of the nozzle is connected to a liquid fuel pressure reducer having relatively large cross sectional passageways for reducing the pressure of the liquid fuel supplied from a manifold. The discharge nozzle includes a liquid fuel atomizing core which employs an atomizing plate together with an air flow entering the nozzle interior through apertures in the nozzle housing from within the hollow post. For gaseous fuel operation of the burner the gas is passed through the hollow post and the bores in the nozzle housing for a discharge towards the combustion chamber. Suitable air inlets between the nozzle discharge end and the combustion chamber are provided for admixing the discharged fuel with sufficient air to sustain the growth of the jet issuing from the nozzle. The air plenum is so constructed that it induces eddy currents within the combustion chamber.

United States Patent Vosper 1451 Aug. 8, 1972 [54] DUCT BURNER FOROPERATION WITH LIQUID OR GASEOUS FUELS [72] Inventor: Ralph R. Vosper,San Jose, Calif.

[73] Assignee: Coen Company, Inc., Burlingame,

Calif.

[22] Filed: Oct. 26, 1970 [21] Appl. No.: 84,036

52 us. c1 ..263/19 A, 431/351, 239/399, 239/432, 239/433, 128/42 51 Int.01 ..F23l 9/04 [58] Held 61 Search.....263/l9 A; 431/285, 351 353;138/42 [56] References Cited UNITED STATES PATENTS 2,318,206 5/1943Eisenlohr ..138/42 x 3,258,056 6/1966 Martin ..263/l9 A 3,273,621 9/1966Childree ..263/l9 A 3,319,947 5/1967 Truesdell. ..263/l9 A 3,393,9647/1968 Donnelly ..431/351 x 3,494,712 2 1970 Vosper et a1 ..263/l9 APrimary Examiner-Carroll B. Dority, Jr. Attorney-Townsend & Townsend[57] ABSTRACT An air augmented liquid or gaseous fuel burner having ahollow post mounting a plurality of fuel discharge nozzles spaced overthe length of the post and a combustion chamber aligned with eachdischarge nozzle. An air plenum is disposed about the nozzle, the postand the combustion chamber. An upstream end of the nozzle is connectedto a liquid fuel pressure reducer having relatively large crosssectional passageways for reducing the pressure of the liquid fuelsupplied from a manifold. The discharge nozzle includes a liquid fuelatomizing core which employs an atomizing plate together with an airflow entering the nozzle interior through apertures in the nozzlehousing from within the hollow post. For gaseous fuel operation of theburner the gas is passed through the hollow post and the bores in thenozzle housing for a discharge towards the combustion chamber. Suitableair inlets between the nozzle discharge end and the combustion chamberare provided for admixing the discharged fuel with sufficient air tosustain the growth of the jet issuing from the nozzle. The air plenum isso constructed that it induces eddy currents within the combustionchamber.

13 Claims, 5 Drawing Figures United States Patent Vosper 1 Aug. 8, 1972PATENIEmus 8 I972 3,682.45 1

sum 1 or 2 FIGJ 8 1 JIIII J I INVENTOR. B 1 Y J 1 BYRALPH R. VOSPERATTORNEYS PATENTEDAuc 8:912

SHEET 2 or 2 W QI m9 mm mm v v: /fi mm om mm 8.

INVENTOR.

R. VOSPER ATTORNEYS RALPH DUCT BURNER FOR OPERATION WITH LIQUID ORGASEOUS FUELS BACKGROUND OF THE INVENTION This invention relates toburners and particularly to those suited for the heating of large air orgas volumes.

Large air volumes are presently heated with so-called duct burnersplaced in air streams for heating the air to the desired temperature. Asa result of the large air volume, which would cool the burner fuel belowits flame point, duct burners are constructed so that they sustain aflame in spite of the excess air volume. Prior art duct burners wereeither gas or oil operated, or they employed both fuel typessimultaneously. Alternative operation of prior art duct burners withliquid or gaseous fuel in an economic manner without at least asubstantial increase in the initial cost of the burners was generallynot possible.

SUMMARY OF THE INVENTION The present invention provides a duct burnerfor the alternative use with liquid or gaseous fuel. The duct burner isefficient and employs the same burner components for both operationalmodes.

Briefly, the duct burner of the present invention comprises a combustionchamber disposed in a duct which has an opening facing in a downstreamdirection of the duct. Fuel discharging nozzle means are disposedupstream of the combustion chamber and interconnected therewith by apassage. Means disposed up stream of a fuel discharge end of the nozzlemeans introduces a gas stream into the interior of the nozzle means forthe subsequent discharge of such gas stream through the discharge endtowards the combustion chamber. Means is provided for supplying oxygento the combustion chamber for at least partial oxidation of the gas orliquid fuel. The burner is liquid fuel operated by passing the liquidfuel through the nozzle means and atomizing it in an air flow enteringthe nozzle means interior through the gas stream introduction stantiallyreduced or eliminated. A tubular air plenum embraces the combustionchamber, the upright posts and the fuel discharge nonle, suppliescombustion air and cools the burner components during operation. Aforward downstream end of the plenum includes openings for the dischargeof flames from the combustion chamber into the duct. The plenum isfurther so constructed that eddy currents are induced at the forward endof the combustion chamber. The eddy currents recirculate burningmaterials from adjacent the forward discharge end of the combustionchamber towards the-base thereof. The combustion of the fuel in thechamber is thereby made substantially more thorough and uniform. Thissubstantially enhances the subsequent combustion of unburned fuelparticles leaving the chamber. An increased burner efficiency and/ordecreased fuel consumption is thereby obtained.

means. It is operated with gaseous fuel by passing gasewhich mounts aplurality of serially arranged discharge I nozzles. The nozzles comprisea housing including apertures communicating the post interior with thehousing interior and an interior nozzle core which combines a liquidfuel flow with the air flow entering through the housing for atomizationof the liquid fuel and subsequent discharge from the nozzle towards thecombustion chamber. For gaseous fuel operation, the gas fuel is passedthrough the hollow post interior and the nozzle housing apertures intothe noule interior and then discharged from the nozzle to the combustionchamber. A liquid fuel restrictor communicating an upstream nozzle endwith a liquid fuel distribution manifold reduces the fuel pressurebetween the manifold to the nozzle by providing a plurality ofrelatively large cross sectional but sharply angularly inclinedpassageways for the fuel. I-Ieretofore often encountered clogging ofrestrictive, pressure reducing apertures in liquid fuel duct bumersarethereby sub- The duct burner of the present invention employs relativelylow cost components which are simultaneously used for its liquid orgaseous fuel operation. It

thus permits the operator to employ whatever fuel is most convenient, orleast expensive at the time to help minimize operating costs. Moreover,the simple, rugged components assure a long and trouble-free servicelife of the burner.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary plan view of avertically installed duct burner constructed in accordance with theinvention;

FIG. 2 is an enlarged fragmentary elevation, with partslbroken away, oftheduct burner illustrated in FIG.

FIG. 3 is an enlarged cross sectional view of a duct burner constructedin accordance with the invention;

FIG. 4 is an enlarged cross sectional view of a liquid or gaseous fueldischarge nozzle constructed in accordance with the invention; and

FIG. 5 is an enlarged cross sectional view of an oil pressure reducingrestrictor coupled with the noule illustrated in FIG. 4 and constructedin accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 through 3,a duct burner 8 for use with liquid fuels, such as oil, or gaseousfuels, such as natural gas, is shown to comprise a plurality ofhorizontally spaced, vertically extending burner columns 10 disposed inside by side relation in a generally horizontal air duct 12.

Upstream end 14 of each column is. closed while. a forward or downstreamend 16 of each column is open for the discharge of flames generated in acombustion chamber 18 through a flame discharge 20 of the chamber tothus heat the air flow in duct 12.

Each burner column 10 comprises an air plenum 22 of a generallyrectangular cross section which extends over the height of duct 12 andis constructed of a plurality of relatively short plenum sections 24arranged end to end and joined withsuitable bands 26. This permits theextension and contraction of the relatively thin walled plenum withoutcausing buckling, warping or the like-under the substantial temperaturedifferentials encountered during operation of duct burner 8.

For assembly purposes the plenum is constructed of a Umshaped aft half28 joined to an inverted, also U- shaped forward half 30 by laterallyextending bolts 32 secured to an upstanding hollow post or pipe 34. Aplurality of washers 36 enable the tightening of nuts 38 withoutdeformation of the plenum halves while they assure correct and equalspacing between the exterior of post 34 and the plenum halves. A web 40of the forward plenum half defining the forward end of burner columnincludes relatively large diameter apertures 42 forming a portion offlame discharge opening 20.

Combustion chamber 18 is vertically continuous and defined by an uprightU-shaped member 44 which is open on the downstream side and disposedadjacent to the forward end of air plenum 22. The U-shaped plenum memberis also constructed of sections 46 which are joined end to end andinterconnected by bands 48 to permit their free expansion andcontraction under the encountered temperature difierentials.Intermittently spaced blocks 50 are secured, e.g., welded, to exteriorsides of U-shaped member 44 adjacent the forward end thereof to maintainan equal spacing between the exterior sides of the U-shaped member andthe interior sides of plenum 22. This permits a free air flow in thatspace towards flame discharge opening 20.

During operation fuel combustion air is blown into the aft portion ofplenum 22 and passes forward between the interior side of the plenum andthe exterior of the combustion chamber. Chamber 18 causes a relativelyhigh velocity hot gas-flame flow from adjacent a base 52 of the chamberthrough flame discharge opening into the air stream in duct 12. Thishigh velocity mixture causes low pressure areas along the interior sidesof U-shaped member 44 and thus draws fresh air from the plenum towardsthe base along arrows 54 of FIG. 3. Good turbulance is thereby obtainedin the combustion chamber, yet unburned fuel particles are returned tothe base and a more uniform combustion of the air-fuel mixture in thechamber is attained. This substantially facilitates the subsequentcomplete combustion of the fuel when admixed with air in duct 12.

A plurality of vertically spaced fuel nozzles 56 are aligned with flamedischarge openings 20, extend diametrically across hollow post 34 andare secured to that post. Pipe sections 58 are secured to post 34concentrically with nozzles 56 and to sections 46 of U- shaped member 44to communicate discharge ends 60 of the nozzle with the combustionchamber through fuel inlet openings 62 in base 52. The pipe sectionsinclude a plurality of circumferentially spaced air inlet apertures 64for admixing fuel discharged by the nozzle with a sufficient amount ofair to sustain the growth of the fuel jet issuing from the nozzle.

Referring to FIGS. 3 through 5, for use of the duct burner with oil orsimilar liquid fuels an oil supply line or manifold 66 is disposedadjacent aft end 14 of each column 10 and extends vertically within airplenum 22. An oil restrictor 68 connects each nozzle 56 with the oilsupply manifold for feeding oil to an upstream or aft end 70 of thenozzle.

Oil pressure restrictors 68 are interposed to reduce the pressure of oilentering nozzle 56 from the pressure prevailing in supply manifold 66.This is necessary manifold. When the burner is operated for low heatoutput, which requires a corresponding reduction of the oil pressure inmanifold 66, there might be insufficient pressure in the manifold tosupply the upper nozzles with any oil whatsoever. Oil pressure reducingdevices are therefore used so that the pressure in the supply manifoldcan substantially exceed the desired oil pressure at the upstreamentrance to the nozzle.

In the past such oil restrictors comprised orifice plates having a smalldiameter aperture through which the oil had to pass in order to reachthe nozzle. The required apertures were so small that they tended toclog easily. To prevent such clogging of the nozzle oil supply therestrictor of the present invention comprises an outer pipe 72 having acylindrical interior surface 74 and, disposed within the pipe, a snuglyfitting oil pres sure reducing core 76. The core extends oversubstantially the full length of the pipe and includes a plurality ofaxially spaced annular grooves 78. Passageways are inclined with respectto the axis of core 76, preferably at an angle of about 60 and theyalternately extend in opposite directions so that the passagewaysterminating at any given annular groove extend nonparallel away fromthat groove. Thus, the annular grooves and the passageways define acontinuous yet tortuous path for the oil for an upstream end ofrestrictor 68 to the downstream end thereof adjacent nozzle intake 70.The nonlinear path and the many directional changes the oil must followto travel over the length of the restrictor causes a substantialpressure drop. With a restrictor of a length of only 1% inches, adiameter for core 76 of only three-eighth inches, seven spaced apartannular grooves 78 and passageways 80 extending between the annulargrooves and inclined 60 from the axis of the core, and groove andpassageway widths and depths of one-sixteenth inch an oil pressure dropof about 50 psi was obtained at a flow rate of about 10 gallons per hourof diesel oil. None of the passageways or annular grooves havetransverse directions less than one-sixteenth inch thus virtuallyeliminating the heretofore troublesome clogging of oil pressurereduction devices.

Referring now particularly to FIGS. 3 and 4, nozzle 56 comprises acylindrical outer housing 82 having a greater length than the diameterof tubular post 34. The upstream end 70 of nozzle is defined by a pipeplug 84 including an interior pipe thread 86 for connection to anexterior pipe thread (not shown) on restrictor 68 and including aforwardly extending, relatively small diameter fuel discharge tube 88terminating about midway between ends of nozzle 56. The forward end oftube 88 extends into a rearwardly and outwardly flaring, cylindricalfuel-air mixing chamber 90 defined by a sleeve 92 threaded into theinterior of housing 82. A liquid fuel atomizing whirl plate 94 includesa plurality of circumferentially arranged apertures 96 and is restrainedto an interior cylindrical portion 98 of housing 82 by the forward endof sleeve 92 and an aft end of an atomized fuel flow tube 100 fittedinteriorly of the housing and extending forwardly to adjacent nozzledischarge 60 within stepped down, cylindrical housing in portion 102.

The downstream end of flow tube 100 has. an exit aperture 104 alignedwith apertures 106 and 108 and an orifice plate 110 and an end cap 112,respectively. The exterior of flow tube 100 is radially spaced fromcylindrical housing end portion 102 and is supported therein by aplurality of circumferentially outwardly extending lugs 114 integrallyconstructed with the flow tube and defining between them passageways116. It will also be noted that the downstream end of flow tube 100 isspaced from the inner side of orifice plate 1 to thus provide acontinuous flow passage from radially outwardly extending, relativelysmall diameter apertures 118 in housing 82, through passageways 116 andto aperture 106 in the orifice plate.

The nozzle housing includes a plurality of outwardly extending,relatively large diameter second apertures 120 which communicate thehousing exterior with the annular space 122 between the housing and theexterior of tube 88 defined by pipe plug 84. This annular spacecommunicates with the outwardly flaring fuel mixing chamber 90.

In operation low pressure liquid fuel, preferably pressurized to onlyabout 5 psi above ambient pressure, is introduced into nozzle 56 andflows through tube 88 into mixing space 90. At the same time lowpressure air is flowed through second apertures 120 and annular space122 towards the mixing chamber. Both the fuel and the air are directedthrough apertures 96in whirl plate 94 and are spun into flow tube 100 sothat the liquid fuel is atomized in the air flow from apertures 120. Theatomized liquid fuel-air mixture flows along tube 100, through tubeaperture 104 and is then discharged from the nozzle through apertures106, 108 in orifice plate 110 and end cap 112, respectively. Pressurizedair is also flowed through first apertures 118 and past passageways 116around the end of flow tube 100 and hence out through the apertures inthe orifice plate and the end cap. This air flow provides a cushionbetween the atomized liquid fuel-air mixture flow and prevents contactbetween the fuel droplets that flow and components of the nozzle. Thecollection of fuel on nozzle components and the clogging of spaces orpassageways with collected fuel, or the dripping of the nozzle, both ofwhich substantially decrease the nozzle efficiency, are thus prevented.

Referring now to FIGS. 1 through 5, nozzle 56 is installed in holes (notshown) of post 34 so that the discharge end 60 of the nozzle is disposedwithin pipe section 58 and upstream intake end 70 of the nozzle projectspast the opposite side of thetubular post. Restrictor 68 is connected tothe nozzle intake end and to an appropriate opening in oil supplymanifold 66. For operation of duct burner 8 with liquid fuel the lowerend of the tubular post 44 and the air plenum 22 Y are connected to asource of pressurized air (not separately shown) and fuel supplymanifold 66 is pressurized for a discharge of an atomized fuel-airmixture by nozzle 56 by virtue of the admixture of the liquid fuel inthe nozzle with pressurized air entering the nozzle interior throughapertures 118 and 120 in housing 82. Additional air is supplied to theatomized fuel-air mixture through pipe section apertures 64. The mixtureis then introduced into the combustion chamber 18 and suitably ignited.The air flow in plenum 22 past the forward end of U-shaped member 44 ofthe combustion chamber induces the above referred eddy currents for auniform combustion of the mixture.

To switch to gaseous fuel operation liquid fuel supply manifold 66 issuitably turned off through the closing of proper valving (not shown)andv simultaneously therewith or thereafter the lower end of tubularpost 34 is connected to a source of pressurized gaseous fuel such asnatural gas. The interior of the tubular post is further sealed from itsexterior. Pressurized gas enters nozzle 56 through apertures 118 and120, while no fuel enters the nozzle from its aft end 70. Alternatively,a liquid-gaseous fuel mixturecan be formed in the nozzle by continuingthe liquid fuel supply while gaseous fuel is passed through the tubularpost. This can also be employed to maintain a continuous flame whileswitching from liquid to gaseous fuel, or vice versa, by temporarilyfeeding both fuels through the nozzle while the tubular post is switchedfrom a gas to an air supply or vice versa. For gaseous fuel operationthe gas is admixed in pipe section 58 with air entering from plenum 22through apertures 64 in the pipe section. The mixture travels forwardinto the combustion chamber where it is ignited as previously described.

I claim:

1. A duct burner for use with a gaseous or a liquid fuel comprising acombustion chamber disposed in a duct and having an opening facing in adownstream direction of the duct, fuel discharging nozzle means disposedupstream of the combustion chamber and having a fuel discharge end,passage defining means interconnecting the nozzle means with thecombustion chamber, the passage defining means comprising a tubularmember, and including an air plenum surrounding the tubular member, thetubular member having at least one opening adjacent the nozzle dischargefor the introduction of air into the fuel flow from the nozzle means tothe combustion chamber, means mounting the nozzle means, means disposedupstream of the fuel discharge end of the nozzle means for theintroduction of a gas stream into the interior of the nozzle means andsubsequent discharge of such gas stream through the discharge endtowards the combustion chamber, means disposed upstream of the dischargeend for supplying the noule with liquid fuel for the subsequentdischarge of the liquid fuel through the discharge end towards thecombustion chamber, and means for supplying oxygen to the combustionchamber for at least partial oxidation of the fuel, whereby the burneris operated by passing liquid fuel through the nozzle means andatomizing it in an air flow entering the nozzle means interior throughthe gas stream introduction means, or by passing gaseous fuel into thenozzle means interior from the gas stream introduction means.

2. Apparatus according to claim 1, wherein said air plenum encloses thecombustion chamber, the nozzle means and the passage defining means forsupplying an air flow to the combustion chamber, the air plenumincluding an opening substantially aligned with the passage definingmeans for exit of combustion chamber games into the duct and heating thegas stream in the net.

3. Apparatus according to claim 2 wherein the air plenum defines a boxsection, and including a plurality of end to end aligned plenum boxsections and means interconnecting the box sections, the interconnectingmeans permitting limited relative longitudinal movement of the boxsections to prevent their buckling under temperature variations.

4. Apparatus according to claim 1, including pressure reducing meansbetween the nozzle means and a liquid fuel supply conduit.

5. Apparatus according to claim 4, wherein the pressure reducing meanscomprises means defining a plurality of angularly disposed, relativelylarge cross section passageways causing a substantial pressure dropbetween an upstream and a downstream end of the passageways.

6. Apparatus according to claim wherein the passageway defining meanscomprises a tubular member, and a snugly fitting core member disposedinteriorly of the tubular member and having a plurality of axiallyspaced annular spaces and a plurality of inclined groovesinterconnecting the annular spaces, grooves terminating at the sameannular space extending nonparallel away from the annular space.

47. Apparatus according to claim 6, wherein the grooves extending from agiven annular space have an angular inclination with respect to eachother of no more than about 90.

8. Apparatus according to claim 1, wherein the nozzle means holdingmeans comprises a tube means supporting the nozzle means between endsthereof, and wherein the noule means includes a plurality of openingscommunicating the tube means with an interior portion of the nozzlemeans for atomization of the liquid fuel within the nozzle means anddischarge of the atomized fuel from the fuel discharge end.

9. Apparatus according to claim 8, wherein the nozzle means includesorifice defining means at the fuel discharge end, and including meanssubstantially preventing contact between the atomized liquid fuel flowpast the discharge end and the orifice defining means.

10. Apparatus according to claim 9, wherein the contact preventing meanscomprises an opening communicating an interior portion of the nozzlemeans adjacent the fuel discharge end with the tube means, the nozzlemeans including means for substantially equally distributing a gas flowabout the periphery of the atomized liquid fuel flow passing the fueldischarge end to thereby prevent direct contact between the orificedefining means and the atomized liquid fuel flow and the formation offuel droplets on the nozzle means.

11. A duct burner for alternative use with a gaseous or a liquid fuelcomprising: a plurality of fuel discharge nozzles mounted to aninterconnecting tubular post for placement of the noules across a duct,the discharge nozzles having an outer housing extending across thetubular post and a plurality of bores communicating the interior of thetubular post with a housing interior, the nozzle further including coremeans permitting the passage of a gas from the post interior through thebores and past a downstream nozzle end, a combustion chambercommunicating with the downstream nozzle end and having an opening forthe discharge of flames into the duct, an augmenting air plenum disposedabout the tubular post, the nozzles and the combustion chamber for thesupply of combustion air and sustaining a flame in the chamber, theplenum including at satzisrzt ssmsssisiafi asrirms the fuel flow betweenthe downstream nozzle end and the combustion chamber, whereby the ductheater can be operated with gaseous fuel by passing such fuel throughthe post interior and the housing interior to the combustion chamber orwith liquid fuel by passing the liquid fuel through the housing interiorfrom an upstream nozzle end and mixing the liquid fuel flow in thehousing interior with and atomizing the liquid fuel flow in air passedinto the housing interior from the post interior through the borescommunicating the post interior with the housing interior.

12. Apparatus according to claim 11, including liquid fuel pressurereduction means disposed between a liquid fuel supply manifold and thenozzles for preventing combustion affecting fluid pressure variations inthe nozzles.

13. Apparatus according to claim 11, wherein the noule core meanscomprise a liquid fuel discharge member disposed coaxially in thehousing interior, means for passing interiorly discharged liquid fueland air from the post interior in a forward direction, an atomizer platedisposed downstream of the interior discharge member for atomizing theliquid fuel in the air flow, means for passing the atomized liquid flowtowards the downstream nozzle end, and means for isolating the atomizedliquid fuel flow and preventing it from contacting nozzle portionsadjacent a downstream nozzle end to prevent the formation of liquid fueldroplets.

1. A duct burner for use with a gaseous or a liquid fuel comprising acombustion chamber disposed in a duct and having an opening facing in adownstream direction of the duct, fuel discharging nozzle means disposedupstream of the combustion chamber and having a fuel discharge end,passage defining means interconnecting the nozzle means with thecombustion chamber, the passage defining means comprising a tubularmeMber, and including an air plenum surrounding the tubular member, thetubular member having at least one opening adjacent the nozzle dischargefor the introduction of air into the fuel flow from the nozzle means tothe combustion chamber, means mounting the nozzle means, means disposedupstream of the fuel discharge end of the nozzle means for theintroduction of a gas stream into the interior of the nozzle means andsubsequent discharge of such gas stream through the discharge endtowards the combustion chamber, means disposed upstream of the dischargeend for supplying the nozzle with liquid fuel for the subsequentdischarge of the liquid fuel through the discharge end towards thecombustion chamber, and means for supplying oxygen to the combustionchamber for at least partial oxidation of the fuel, whereby the burneris operated by passing liquid fuel through the nozzle means andatomizing it in an air flow entering the nozzle means interior throughthe gas stream introduction means, or by passing gaseous fuel into thenozzle means interior from the gas stream introduction means. 2.Apparatus according to claim 1, wherein said air plenum encloses thecombustion chamber, the nozzle means and the passage defining means forsupplying an air flow to the combustion chamber, the air plenumincluding an opening substantially aligned with the passage definingmeans for exit of combustion chamber flames into the duct and heatingthe gas stream in the duct.
 3. Apparatus according to claim 2 whereinthe air plenum defines a box section, and including a plurality of endto end aligned plenum box sections and means interconnecting the boxsections, the interconnecting means permitting limited relativelongitudinal movement of the box sections to prevent their bucklingunder temperature variations.
 4. Apparatus according to claim 1,including pressure reducing means between the nozzle means and a liquidfuel supply conduit.
 5. Apparatus according to claim 4, wherein thepressure reducing means comprises means defining a plurality ofangularly disposed, relatively large cross section passageways causing asubstantial pressure drop between an upstream and a downstream end ofthe passageways.
 6. Apparatus according to claim 5 wherein thepassageway defining means comprises a tubular member, and a snuglyfitting core member disposed interiorly of the tubular member and havinga plurality of axially spaced annular spaces and a plurality of inclinedgrooves interconnecting the annular spaces, grooves terminating at thesame annular space extending non-parallel away from the annular space.7. Apparatus according to claim 6, wherein the grooves extending from agiven annular space have an angular inclination with respect to eachother of no more than about 90*.
 8. Apparatus according to claim 1,wherein the nozzle means holding means comprises a tube means supportingthe nozzle means between ends thereof, and wherein the nozzle meansincludes a plurality of openings communicating the tube means with aninterior portion of the nozzle means for atomization of the liquid fuelwithin the nozzle means and discharge of the atomized fuel from the fueldischarge end.
 9. Apparatus according to claim 8, wherein the nozzlemeans includes orifice defining means at the fuel discharge end, andincluding means substantially preventing contact between the atomizedliquid fuel flow past the discharge end and the orifice defining means.10. Apparatus according to claim 9, wherein the contact preventing meanscomprises an opening communicating an interior portion of the nozzlemeans adjacent the fuel discharge end with the tube means, the nozzlemeans including means for substantially equally distributing a gas flowabout the periphery of the atomized liquid fuel flow passing the fueldischarge end to thereby prevent direct contact between the orificedefining means and the atomized liquid fuel flow and the formation offuel droplets on the nozzle means.
 11. A duCt burner for alternative usewith a gaseous or a liquid fuel comprising: a plurality of fueldischarge nozzles mounted to an interconnecting tubular post forplacement of the nozzles across a duct, the discharge nozzles having anouter housing extending across the tubular post and a plurality of borescommunicating the interior of the tubular post with a housing interior,the nozzle further including core means permitting the passage of a gasfrom the post interior through the bores and past a downstream nozzleend, a combustion chamber communicating with the downstream nozzle endand having an opening for the discharge of flames into the duct, anaugmenting air plenum disposed about the tubular post, the nozzles andthe combustion chamber for the supply of combustion air and sustaining aflame in the chamber, the plenum including at least one aperturepermitting flames from the opening to discharge into the duct, and meansfor adding air to the fuel flow between the downstream nozzle end andthe combustion chamber, whereby the duct heater can be operated withgaseous fuel by passing such fuel through the post interior and thehousing interior to the combustion chamber or with liquid fuel bypassing the liquid fuel through the housing interior from an upstreamnozzle end and mixing the liquid fuel flow in the housing interior withand atomizing the liquid fuel flow in air passed into the housinginterior from the post interior through the bores communicating the postinterior with the housing interior.
 12. Apparatus according to claim 11,including liquid fuel pressure reduction means disposed between a liquidfuel supply manifold and the nozzles for preventing combustion affectingfluid pressure variations in the nozzles.
 13. Apparatus according toclaim 11, wherein the nozzle core means comprise a liquid fuel dischargemember disposed coaxially in the housing interior, means for passinginteriorly discharged liquid fuel and air from the post interior in aforward direction, an atomizer plate disposed downstream of the interiordischarge member for atomizing the liquid fuel in the air flow, meansfor passing the atomized liquid flow towards the downstream nozzle end,and means for isolating the atomized liquid fuel flow and preventing itfrom contacting nozzle portions adjacent a downstream nozzle end toprevent the formation of liquid fuel droplets.